This invention generally relates to brazing methods, including processes and materials for use in the manufacturing, coating, repair, and build-up of superalloy components. More particularly, this invention relates to a method of brazing a superalloy, in which microwave energy is employed to melt a braze alloy that contains minimal or no melting point depressants relative to the superalloy being brazed.
Nickel, cobalt, and iron-base superalloys are widely used to form high temperature components of gas turbine engines. While some high-temperature superalloy components can be formed as a single casting, others are preferably or required to be fabricated by other processes. As an example, brazing is widely used to fabricate gas turbine components, as in the case of high pressure turbine nozzle assemblies. Brazing techniques conventionally encompass joining operations performed at an elevated temperature but sufficiently below the melting points of the superalloys being joined so as not to cause grain growth, incipient melting, recrystallization, or unfavorable phase formation that can lead to degradation of the alloys. In carrying out the brazing process, an appropriate braze alloy is placed between the interface (faying) surfaces to be joined, and the faying surfaces and the braze alloy therebetween are heated in a vacuum to melt the braze alloy. The braze alloy melts at a lower temperature than the superalloy base material as a result of containing one or more melting point depressants, such as boron and/or silicon in an amount greater than the superalloy(s) being brazed. For example, whereas superalloys containing intentional amounts (above impurity levels) of boron and/or silicon typically contain less than 0.1 weight percent of each, braze alloys that contain boron and/or silicon as melting point depressants typically contain at least 2.0 weight percent boron, or at least 6.0 weight percent silicon, or both silicon and boron at a ratio of about 3:1. On cooling, the braze alloy solidifies to form a permanent metallurgical bond.
During engine operation, gas turbine engine components are subject to strenuous high temperature conditions under which various types of damage or deterioration can occur. As examples, erosion and oxidation reduce wall thicknesses of turbine nozzles and vanes, and cracks can initiate at surface irregularities and propagate as a result of stresses that are aggravated by thermal cycling. Because the cost of components formed from superalloys is relatively high, it is often more desirable to repair these components rather than replace them. In response, brazing techniques have been developed for crack repair and wall thickness build-up that entail placing a braze alloy filler metal on the surface area requiring repair, and then heating the filler metal in a vacuum to above its melting point, but below that of the surface substrate, so that the molten filler metal wets, flows, and fills the damaged area.
While widely employed to fabricate and repair gas turbine engine components, conventional brazing processes have notable disadvantages. For example, the entire component must be subjected to a vacuum heat treatment, which is a very lengthy process in a production environment, unnecessarily exposes undamaged regions of the component to high temperatures, and can potentially remelt joints in other sections of the component. Furthermore, though braze alloys typically have compositions similar to the base metal of the component being brazed, the presence of boron and/or silicon in a braze alloy as a melting point suppressant reduces the mechanical and environmental properties of the resulting brazement as a result of the minimal ductility of the borides and silicides they form by reaction with refractory elements. Boron and silicon can also diffuse into the base metal repaired by the brazement to adversely affect the mechanical and environmental properties of the component.
Microwave brazing has been investigated as a potential candidate for eliminating these issues, as heating can be localized to selected areas of a component. The general approach has been to use a susceptor (e.g., SiC enclosure) that is heated when exposed to microwave energy and, in turn, transfers the heat to the component by radiation. Drawbacks include the lack of local heating of the braze alloy only, as an entire region of the component is inevitably heated, and significant heat loss from radiation in directions away from the intended brazement.